激光拉曼原位观测储层温度压力条件下乙烷在纯水中的扩散系数

Determination of Diffusion Coefficients of Ethane in Water at High Pressure and Temperature with In-Situ Raman Spectroscopy

乙烷和甲烷是深部天然气藏中的重要组成部分,精确测量其扩散系数对于正确理解深部页岩储层中的烃类气体的分布运移及其分异具有重要意义.目前乙烷在水中的扩散系数数据局限在低压和283-333K温度范围内,缺少天然气储层高温高压条件下的数据.本研究利用显微激光拉曼光谱,在高压透明毛细管中原位观测了20MPa下273-393K温度范围内乙烷在纯水中的扩散,测定了各温度下的扩散系数,并用Speedy-Angell指数方程拟合出乙烷扩散系数D（乙烷）（m^2/s）与温度T（K）之间的关系式：D（C2H6）=D0[（T/Ts）-1]^γ,式中：D0=13.805 5×10^-9 m^2/s,Ts=237.4K,γ=1.739 7.相同温度压力条件下,测得的乙烷的扩散系数小于甲烷的扩散系数.据此计算了2种气体通过低渗透盖层的扩散量的差异,发现甲烷和乙烷溶解扩散的分异程度随盖层厚度、扩散时间而显著变化.

Ethane and methane are two main components of natural gas.Accurate diffusion coefficients at high temperature for ethane and methane are the key to calculate the diffusion and fractionation of gases in deep natural gas reservoirs,especially in unfractured shale and other geological materials with low permeability.However,the diffusion coefficients for ethane at pressures and temperatures near the reservoir conditions are scarce in the literature.In this study,diffusive transfer of ethane in water at 20 MPa and from 273 to 393K was observed in a high-pressure optical capillary cell via Raman spectroscopy.Diffusion coefficients were then determined by the least-square method.The relationship between diffusion coefficients[D（C2H6）in m^2/s]and temperature（Tin K）was derived with Speedy-Angell power-law equation as：D（C2H6）= D0[（T/Ts）-1]^γ,where D0 =13.805 5×10^-9 m^2/s,Ts=237.4K,γ=1.739 7.The diffusion coefficients of ethane are much smaller than those of methane at the same condition.The amount of methane and ethane diffused through some thick low permeability layers are calculated and the results show that such diffusivity difference can cause a significant fractionation of methane and ethane in thick shale layer.